Filter on nameplates debuffs.

[Vrul]

I don't see a way to do it without taint but this didn't throw any errors the little I tested at a training dummy:

local GetNamePlateForUnit = C_NamePlate.GetNamePlateForUnit
local OriginalUpdateBuffs = NameplateBuffContainerMixin.UpdateBuffs
 
local alteredNamePlates = { }
 
local function ShouldShowBuff(frame, name, caster)
    return name -- already filtered
end
 
local function UpdateBuffs(self, unit, filter, showAll)
    OriginalUpdateBuffs(self, unit, "HARMFUL|PLAYER")
end
 
local frame = CreateFrame("Frame")
 
frame:SetScript("OnEvent", function(self, event, unit)
    local nameplate = GetNamePlateForUnit(unit)
    if nameplate and not alteredNamePlates[nameplate] then
        alteredNamePlates[nameplate] = true
        local buffFrame = nameplate.UnitFrame.BuffFrame
        buffFrame.ShouldShowBuff = ShouldShowBuff
        buffFrame.UpdateBuffs = UpdateBuffs
    end
end)
 
frame:RegisterEvent("NAME_PLATE_UNIT_ADDED")